The present invention relates generally to offset printing presses, and in particular to method and apparatus for reducing infiltration of dampening solution in the inking system of a printing press.
In lithographic printing, a web printing press utilizes a plate cylinder, a lithographic printing plate, a blanket cylinder. The printing plate surface is chemically treated to form mutually exclusive ink receptive areas and ink repellant areas. Ink is transferred to the printing plate by an inking roller train.
The lithographic plate surface has non-image areas which are hydrophilic (wettable by water) and also has image surface areas which are oleophilic (wettable by ink). The wettable surfaces of the lithographic plate are completely wetted with a water film (a dampening solution) applied by dampening rollers, and a lithographic oil-based ink is then applied to the printing plate by inking rollers. The hydrophobic oil-based ink is rejected by the hydrophilic surface areas that are wetted by aqueous dampening solution. This produces an inked image on the oleophilic surface of the printing plate which is then transferred to the surface of an ink receptive blanket, which transfers (offsets) the inked image onto a substrate, for example a paper web.
In lithographic printing processes, printing ink is supplied from a reservoir or fountain by an ink ductor roller which transfers ink to the inking roller train, which in turn transfers it to the printing plate. The inking roller train includes storage rollers for milling the ink to obtain a desired fluidity, vibrator rollers which reciprocate axially to distribute the printing ink uniformly, idler storage rollers, bridging rollers and form rollers which apply the milled ink to the printing plate.
Dampening solution is supplied from a reservoir or fountain by a dampener roller train which transfers it to the printing plate. The dampener roller train typically includes a pan roller, a water transfer roller, a dampening form roller and one or more distribution rollers. The dampener roller train is installed on the rear (feeder) side of the plate cylinder and operates in parallel with the inking roller train.
In lithographic printing, the inking form rollers in contact with the printing plate pick up some of the dampening solution from the plate and transfer some of it to other rollers in the inking roller train. The infiltration of dampening solution into the inking roller system adversely affects the printed image by causing image defects known in the printing trade as "wash-out" and "ghosting." Excess dampening solution also reduces the color intensity of the ink, makes the ink less scratch resistant and retards drying of the ink.
Planographic printing processes including lithographic printing are widely used for producing a myriad of printed materials and in particular in multicolor printing. In the basic process of lithographic printing, an aqueous dampening solution is applied to the plate cylinder in order to prevent the deposition of ink on the printing plate in the areas where no image is wanted. Conventional lithographic printing presses utilize a relatively complex system of inking rollers to distribute the ink evenly across the plate cylinder. The printing plate must, of course, also be in contact with a dampener system which normally includes a set of dampener rollers engaged with the printing plate at a location on the feeder side of the plate cylinder which is generally diametrically opposite to the inking form rollers. However, inevitably, the inking form rollers in contact with the printing plate pick up some of the water from the printing plate, and the water then infiltrates through the inking roller train.